soma: P3 cleanup — LinkPreview a11y section + TagGroup key API consistency

- LinkPreview README: elevate the "intentionally hidden from AT" note to
  its own `## Accessibility` section with the three reasons (duplicate
  content, hover-only activation, focus stays on trigger). Consumers
  reaching for an AT-visible preview are pointed at Popover / Tooltip.
- TagGroup `handleItemKeydown`: vertical nav now calls
  `getDirectionalKeys(dir, 'vertical')` (returns UP/DOWN absolute) instead
  of hardcoded `KEYS.ARROW_DOWN/UP`. Functionally identical — vertical
  keys don't flip under RTL — but keeps the shape symmetric with the
  horizontal branch and matches the convention used across the library.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/uix/soma/components/link-preview/README.md

@@ -30,7 +30,28 @@ A floating preview card that appears on hover over a link. Supports configurable
 | ------- | ------------- | ------ |
 | Content | `aria-hidden` | `true` |
 
-The preview content is intentionally hidden from assistive technology — it is a visual convenience only. The underlying link remains fully accessible.
+## Accessibility
+
+**LinkPreview is invisible to assistive technology by design.** The floating
+preview card exists for sighted-hover discovery only — it adds no
+information a screen-reader user could not already reach through the
+underlying `<a>` element. Emitting `aria-hidden="true"` on `Content` is a
+deliberate choice:
+
+- **Duplicate content:** the preview typically mirrors the destination page
+  summary that a screen reader would announce after following the link.
+  Re-reading it as a tooltip is noise, not help.
+- **Hover-only activation:** APG explicitly recommends against exposing
+  pure-hover widgets to assistive tech because keyboard / touch users never
+  see them anyway — making them invisible everyone except mouse hover is
+  the consistent experience.
+- **Focus stays on the trigger:** the preview never steals focus, never
+  traps Tab, and closes automatically on blur. There is nothing for a
+  keyboard / AT user to miss.
+
+If your LinkPreview contains content that is NOT in the destination page
+(a custom CTA, a badge, a pricing hint), consider using `Popover` or
+`Tooltip` instead — both are AT-visible.
 
 ## Data Attributes
 

src/uix/soma/components/tag-group/tag-group-provider.svelte.ts

@@ -156,7 +156,11 @@ export class TagGroupProvider extends Provider<TagGroupOpts> {
 	handleItemKeydown(e: SomaKeyboardEvent<HTMLElement>, value: string) {
 		if (this.opts.disabled.current) return;
 		const dir = this.resolvedDir.current;
+		// Horizontal flips under RTL; vertical doesn't (UP / DOWN are absolute
+		// per APG). Using `getDirectionalKeys` for both axes keeps the shape
+		// symmetric — consistent with other components that navigate grids.
 		const { nextKey, prevKey } = getDirectionalKeys(dir, 'horizontal');
+		const { nextKey: downKey, prevKey: upKey } = getDirectionalKeys(dir, 'vertical');
 		const items = this.getItems();
 		if (items.length === 0) return;
 		const currentIndex = items.indexOf(e.currentTarget);
@@ -171,9 +175,9 @@ export class TagGroupProvider extends Provider<TagGroupOpts> {
 			this.focusAt(currentIndex - 1);
 			return;
 		}
-		if (e.key === KEYS.ARROW_DOWN || e.key === KEYS.ARROW_UP) {
+		if (e.key === downKey || e.key === upKey) {
 			e.preventDefault();
-			const delta = e.key === KEYS.ARROW_DOWN ? 1 : -1;
+			const delta = e.key === downKey ? 1 : -1;
 			this.focusAt(currentIndex + delta);
 			return;
 		}